Multi-layer ceramic (MLC) structures are used in the production of electronic substrates and devices. The MLCs can have various layering configurations. For example, a MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched in between ceramic layers which act as a dielectric medium. For the purposes of interlayer interconnections, most of the ceramic layers have tiny holes or via holes. Prior to lamination, the via holes are filled with an electrically conductive paste, such as, a metallic paste, and sintered to form vias which provide the electrical connection between the layers. In addition, the MLC substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, to name a few.
Generally, conventional ceramic structures are formed from ceramic green sheets which are prepared from a slurry of ceramic particulate, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After punching, metal paste screening, stacking and laminating, the green sheets are fired or sintered at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulate together into a densified ceramic substrate. The present invention is directed to the screening, stacking and lamination steps of this process.
In the MLC packaging industry it is very common to use green sheets of various thicknesses. The thicknesses can typically vary from 6 mils to 30 mils and in general the art of punching and metallizing these layers are well known. Green sheet thicknesses below 6 mils, in general, are very scarcely used. This is due to a variety of reasons, such as, for example, handling, screening and stacking of green sheets thinner than 6 mils pose tremendous challenges. In fact the use of one to two mils thick ceramic green sheets, which are punched and screened, using traditional MLC technology does not exist in the prior art.
Also, in the MLC packaging industry it is very common to use capacitor layers. The capacitance necessary in a package depends on the design and such capacitance is obtained by choosing proper dielectric layer thickness and metal area within a layer. The industry is always striving for higher capacitance and since the metal area is maxing out for a given substrate size it is necessary to use thinner dielectric layers between electrodes to obtain the required capacitance. For example, as a rule of thumb one could double the capacitance for a given dielectric system and electrode metal area by decreasing the dielectric layer thickness by half. Additionally the number of layers needed for capacitance in a package as well has been reduced by about 50 percent. The reduction in the number of layers is desirable, as it reduces the cost and the process of making the substrate.
The term thin sheet or layer as used herein means that the thickness of the sheet can be anywhere from about 0.5 mil to about 6.0 mils. Production level screening and stacking of thin sheets is not possible with the current technology as the thin sheets tend to shrink a lot and they also tend to distort during the process.
U.S. Pat. Nos. 5,176,772 and 5,300,163, addresses a method of forming a thin sintered ceramic board by laminating punched and screened green tapes on a presintered ceramic body and sintering the assembly to obtain a flat board. This method essentially enables one to obtain a flat laminate. But, with ceramic substrates with materials like alumina and aluminum nitride there is typically a 14 to 20 percent X-Y shrinkage. So there will be delamination and distortion involved in sintering due to presintered base. Furthermore, one needs to handle these sheets through screening in free standing state.
U.S. Pat. No. 5,368,667, teaches preparing a multi-layer capacitor which is one to two mils thick by extruding a thick dielectric layer and a metal containing layer through a roll laminator. This approach is good for using a thin layer having the blanket metallurgy. But in almost all multi-layer ceramic packages, one needs to have vias for electrical connectivity. However, this patent does not address the problem of handling the thin sheets in via filling and stacking.
U.S. Pat. No. 5,480,503, teaches releasably-supporting the thin green sheets on a temporary carrier support having an ablatable release layer over a patterned conductive layer, and filling the vias with conductive metal paste, whereby the thin green sheets are supported against warpage and distortion. The supported green sheets are formed as single layers, pairs and stacks thereof, and separated from temporary support for use. The suggested temporary support is a glass plate. The metallization technique is CVD type plating and in the process has to use non-ablatable and ablatable films on the green sheet. These films inherently will distort the green sheet during temperature and pressure processing. Further the non-ablatable film stays with every single green sheet layer and will create delamination and density difference in sintering. This is a very complicated and very expensive process.
The structure and method of the present invention enables the screening, stacking and handling of very thin green sheets and/or green sheets with very dense metallized patterns in the manufacture of multi-layer ceramic packages. With the preferred embodiment, thin punched green sheets were tacked and bonded to thick punched and screened green sheets to form a sub-structure which yielded excellent stability in screening and enabled excellent handling and alignment in stacking. The green sheet may have electrically conductive features within them, such as, a via, or over them, such as, a line, cap, to name a few.